This invention relates to an optical line terminal, and more particularly, to an optical line terminal for allocating a bandwidth to an optical network unit.
Optical networks are being introduced in order to address an increase in speed and bandwidth of communication networks. A passive optical network (hereinafter, referred to as PON) is proposed as the optical network to be introduced.
The PON is a system in which one of a station-side optical line terminal (hereinafter, referred to as OLT), an optical fiber, an optical splitter for branching the optical fiber, and a plurality of indoor optical network units (hereinafter, referred to as ONUs) are coupled via a point-to-multi point network in a star configuration. Representative standards of the PON include the Ethernet (trademark) PON (EPON) standardized by IEEE 802.3, and the gigabit capable PON (GPON) standardized by ITU-T G.984.
An upstream frame transmitted from the ONU toward the OLT and the downstream frame transmitted from the OLT toward the ONU are multiplexed by the wave division multiplexing (hereinafter, referred to as WDM) in the PON.
The OLT transmits the same data by means of a downstream frame to all the ONUs coupled via the optical fiber. The ONU, which has received the downstream frame, refers to the destination information contained in the downstream frame, and discards downstream frames other than downstream frames directed to the ONU itself. Then, the ONU transfers only data directed to the ONU itself contained in the downstream frame directed to the ONU itself to a user side.
Moreover, a system in which the ONU transmits an upstream frame to the OLT at a time point specified by the OLT according to a transmission permission from the OLT is proposed (see, for example, Japanese Patent Application Laid-open No. 2007-019815). The upstream frame contains data multiplexed by the time division multiple access (hereinafter, referred to as TDMA).
Moreover, the PON includes the PON communicating at a low speed of approximately 64 kbit/second, the Broadband PON (BPON) transmitting/receiving fixed-length ATM cells at a speed up to approximately 600 Mbit/second, the EPON transmitting/receiving a variable-length packet of the Ethernet at a speed up to approximately 1 Gbit/second, the GPON carrying out high-speed communication at approximately 2.4 Gbit/second, and the like. An introduction of a PON enabling a high-speed communication out of these PONs is in progress, and realization of a high-speed PON which can communicate at 10 Gbit/second to 40 Gbit/second is sought.
As the communication speed of the PON increases in this way, the power consumption of relay devices on the transmission line also increases. On the other hand, the ONU in the PON is installed in the subscriber's home, and a large number of ONUs are provided on the network. Therefore, the power consumption by the ONUs is increasing.
Moreover, the ONU requires an available bandwidth for a short period compared with the OLT and higher-level switches. Thus, the ONU may be left consuming a wasteful electric power during a non-communication state.
In view of the above-mentioned situation, a need for saving the electric power of the ONU is increasing, and an electric power saving feature for controlling the ONU to transition to a power saving (sleep) state, and also controlling the ONU to transition from the sleep state to an active state in which the upstream frame and the downstream frame can be transmitted and received by means of the control from the OLT is proposed (see, for example, Japanese Patent Application Laid-open No. 2010-114830).